Judicial Watch, a public-interest watch dog group, just unearthed a cache of e- mails showing that Elena Kagan participated in preparing the government’s defense of ObamaCare during her time as President Obama’s solicitor general. By all precedent — and her promise to the Senate Judiciary Committee she’d abide by “letter and spirit” of the law — this should oblige Justice Kagan to recuse herself when the ObamaCare cases come before the Supreme Court.

Before joining the court, Kagan said she was not involved in the Justice Deparment’s preparations for the inevitable legal challenges to the health-care “reform.” But the e-mails suggest otherwise:

* In January 2010, then-Deputy Solicitor General Neal Katyal told Brian Hauck, senior counsel to Associate AG Thomas Perrelli: “Brian, Elena would definitely like OSG [Office of Solicitor General] to be involved in this set of issues [health-care defense] . . . we will bring in Elena as needed.”

* Later, Katyal urged Kagan to attend a health-care-defense meeting.

But once Obama announced he was appointing Kagan to the high court, Katyal changed his tune: He said Kagan had been “walled off” and told Tracy Schmaler, a DOJ spokesperson, that he’d “never discussed the [ObamaCare] issues with [Kagan] one bit.”

Kaytal forwarded Kagan his e-mail to Schmaler telling her: “This is what I told Tracy about Health Care.”

Kagan herself then e-mailed Schmaler: “This needs to be coordinated. Tracy you should not say anything about this before talking to me.”

This is highly suspicious. Why would Kaytal have felt the need to tell Kagan “this” is what he’d told Schmaler, and Kagan to tell Schmaler “this needs to be coordinated,” if they were telling the unvarnished truth?

The Justice Department withheld other relevant e-mails — but was forced to prepare a log describing them. The log shows that Kagan was a recipient and author on e-mail chains under the subject “health care,” discussing “what categories of legal arguments may arise.”

Attorneys general from more than half the states have filed a lawsuit contending ObamaCare’s “individual mandate” exceeds Congress’ power to regulate commerce. In that case, a federal judge ruled the whole law unconstitutional; in other (less significant) cases, federal judges have ruled either for or against upholding the law. The issue is plainly headed to the Supreme Court after appeals.

But the high court last month declined to “fast track” Virginia Attorney General Ken Cucinelli’s case — and there’s no indication Kagan recused herself from discussion of deciding Cucinelli’s request. Yet Kagan herself testified that she’d abide by the law and custom that require her to disqualify herself “in any proceeding in which [her] impartiality might be reasonably questioned . . . or where [she] has served in governmental employment and in such capacity participated as counsel, adviser . . . [or] expressed an opinion concerning the merits.”

Rep. Anthony Weiner (Brooklyn-Queens) and 73 other congressional Democrats recently wrote Justice Clarence Thomas asking him to recuse himself. The reason? Thomas’ wife, Virginia, worked for a conservative think tank, The Heritage Foundation, until 2008 — long before Congress took up ObamaCare. Pretty irrelevant. Conversely, the release of the e-mails makes the case for Kagan’s recusal quite strong.